foo.c

# extconf.rb
require 'mkmf'
create_makefile('foo')

# foo.h
#ifndef __FOO_H_INCLUDED__
#define __FOO_H_INCLUDED__

#include <ruby.h>

extern VALUE cFoo;
extern VALUE cBar;

static VALUE rb_hash_aref2(VALUE v_hash, char* key){
  VALUE v_key, v_val;

  v_key = rb_str_new2(key);
  v_val = rb_hash_aref(v_hash, v_key);

  if(NIL_P(v_val))
    v_val = rb_hash_aref(v_hash, ID2SYM(rb_intern(key)));

  return v_val;
}

#endif

// foo.c
#include <foo.h>

VALUE cFoo;

static VALUE foo_alpha(VALUE v_hash){
  VALUE v_hello = rb_hash_aref2(v_hash, "hello");
  return v_hello;
}

void Init_foo(){
  cFoo = rb_define_class("Foo", rb_cObject);

  rb_define_method(cFoo, "alpha", foo_alpha, 1);

  Init_bar();
}

// bar.c
#include <foo.h>

VALUE cBar;

void Init_bar(){
  cBar = rb_define_class("Bar", rb_cObject);
}